[Xastir] Question about APRS GPS position precision

[Andrew Rich]

Yeah but in the real world, it still puts me off the road or runway.


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Andrew Rich VK4TEC
vk4tec at people.net.au <mailto:vk4tec at people.net.au>
http://www.tech-software.net




-----Original Message-----
From: xastir-bounces at xastir.org [mailto:xastir-bounces at xastir.org]On
Behalf Of Gerry Creager
Sent: Monday, 8 October 2007 6:54 AM
To: Richard Polivka, N6NKO
Cc: Jim Tolbert; XASTIR
Subject: Re: [Xastir] Question about APRS GPS position precision


Richard Polivka, N6NKO wrote:
> Jim,
>
> Most GPS units are good to 4 decimals. Any higher precision requires
> post-processing or L1/L2 reception (not avail in consumer equipment).
> When you factor in multipath and all the other variables, 4 decimals is
> quite good but it takes time and patience - think searching for a
> geocache in a forest. Plus, at four decimals, on a patch antenna minus
> ground plane, it is quite unstable.

9 cm more or less should be plenty good enough for most of our users.
That's 4 decimal-place precision.  That said, an L1 signal (L5 won't be
available for some time still) position assuming really good geometry
and a stable antenna platform is likely to be good only to ~6m
horizontal and ~13.7m vertical... at best.

> Plus, I have a feeling that when Bob B. designed APRS, he was not
> looking at this being used for what we are doing.
>
> Until the data output is smoother and better accuracy, five decimals in
> - broadcast 4 - rewrite the standard, this may be the best for now.

The limitations in precision are in rank order, the spec and the spec.
For accuracy the limitations are:
User equipment antenna configuration
Ionosphere
Troposphere
Multipath
GPS Signal Specification for L1

When I resolve cm accuracy, or better, I do it using dual-frequency
(L1/L2) receivers, multiple stable baseline processing on ground-plane
or choke-ring antennas, at a fixed and measured height about the ground,
and post-process the data to include a least-squares adjustment of the
position.  The process is as much statistical as matrix-mathematical in
accomplishment.

gerry

> Jim Tolbert wrote:
>> Hi, all..........
>>
>> I have been told that the limiting factor in position precision is the
>> APRS system transmission standard-- that transmissions are limited to
>> 1/1000th of a degree or approximately +/- 60 feet.   Is this true?  If
>> so, why?
>>
>> If not, what is the limiting element in the final display of tracker
>> position on Xastir?
>>
>> For those of you using Xastir for Search & Rescue, do you run on the
>> APRS frequency or a different (quiet) frequency?   What are the
>> arguments for each school of thought?   We are going to be running
>> some field tests in the near future with a variety of equipment
>> setups....  does anyone have suggestions of things we should test?  We
>> have a list, but new ideas and comments would be appreciated from
>> those that have already taken the stumbles<grin>.
>>
>> Many thanx.............. jt
>>
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--
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843
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